Inertial sensor systems or inertial navigation units generally include inertial sensors such as accelerometers and gyroscopes, i.e., gyros. The sensors are generally rigidly and precisely mounted within an enclosure along with related electronics and hardware. In turn, the enclosure is rigidly and precisely mounted to a support frame in a vehicle, such as an aircraft, missile, or satellite. Precision mounting of these components is required so that the alignment of the sensor relative to the support frame as well as the enclosure is known, and the sensor outputs are utilized by a system computer as is well known in the art.
The sensor system generally includes a plurality of inertial sensors and a navigational computer. The inertial sensors provide inertial data, such as linear acceleration and rotational velocity or angular information, to the navigational computer which processes the information for a variety of purposes such as flight control, navigation or pointing. For proper performance of a sensor system, the geometrical relationship between each of the inertial sensors must be known, and the relationship between each of the inertial sensors and the vehicle support frame must also be known so that the navigational computer may provide a user with correct information.
Inertial systems data, measured along the input axes of the gyros and accelerometers must be compensated and transformed to coordinates defined by the ultimate user of the sensor system. For optimum performance of inertial sensor systems, precise alignment or orientation of the inertial sensors relative to the vehicle must be known and held to tight tolerances. Most vehicles are provided with a shelf or rack which allows for installation of the sensor systems according to reference edges or surfaces on the enclosure. For example, with a reference surface on the enclosure as well as a reference edge defined, a 3-D axis system can be determined for the inertial sensor system which is transferable to the vehicle. However, before the inertial sensor system can be installed on a vehicle, a 3-D reference system must be established between the exterior of the enclosure and the sensors themselves.
The output data reference frame of the sensors with respect to the enclosure is called the "mounting frame", or M-frame. The definition of the M-frame varies greatly between applications, and is often overlooked until late in the design, or is specified in a casual and inadequate manner. The reason for the difficulty is that the mounting reference involves both the inertial sensor enclosure design, as well as the equipment in which the inertial sensor system is to be installed.
In the past, an M-frame was established through some sort of optical reference, such as an optical cube, mirrors, or a combination of a mirror and a porro prism The disadvantage of using these optical methods is that surveying is required by both the inertial system manufacturer and the customer. High skill levels are required and the process is subject to error. The calibration must be checked and rechecked. Further, there are many situations in which an optical line-of-sight is not available.
Another approach is to use one mounting surface on the enclosure and a mirror as a reference. Although this reduces some of the complex optical measurements required, it does not eliminate them entirely. Therefore, it is the desire to establish a simpler procedure for calibrating an inertial sensor assembly while retaining a high level of accuracy.